The present invention relates to a two component room temperature vulcanizable silicone rubber composition and more particularly, the present invention relates to a completely solventless two component room temperature vulcanizable silicone rubber composition.
Two component room temperature vulcanizable silicone rubber compositions are well known ("room temperature vulcanizable" shall hereinafter in this application be referred to as RTV). Generally, such two component RTV compositions or two part RTV compositions comprise as a basic ingredient a silanol end-stopped diorganopolysiloxane polymer an alkyl silicate or a partial hydrolysis product of an alkyl silicate as a cross linking agent and a tin salt of a carboxylic acid with such tin salt of a carboxylic acid preferably being used as the curing catalyst. Such composition is two component, or two part, in that all the ingredients are not packaged in the same package; that is normally the alkyl silicate is packaged with a tin salt or with the silanol polymer and the tin salt is maintained separate from the silanol polymer. When it is desired to cure the composition, the two packages or two components are mixed and the compositions are allowed to cure at room temperature.
Such compositions may include self-bonding additives such as nitrogen functional silanes, various of fillers, both reinforcing and extending, flame retardant additives, heat agent additives, and oil resistant additives and other types of additives. An example of such a two component RTV composition with self-bonding additives added to it is for instance disclosed in Bessmer et al. U.S. Pat. No. 3,888,815 which is hereby incorporated by reference. Such compositions are also disclosed in which the alkyl silicate is pre-reacted with a tin salt at temperatures in the range of 80.degree.-200.degree. C. for at least 20 minutes and then after being pre-reacted, being utilized in the two component RTV composition. The reasons for pre-reacting the catalyst, as disclosed in Lewis et al. U.S. Pat. No. 3,186,963 which is hereby incorporated by reference, are that the simple mixture of the tin salt with the cross linking agent has a freezing point which is high enough to preclude its use as a liquid catalyst at low temperatures and that the activity of the tin salt and alkyl silicate varies from batch to batch, unless it is pre-reacted as outlined in the foregoing Lewis et al patent.
An improvement over that system is the patent of Smith et al. U.S. Pat. No. 3,957,704 in which the pre-reacted catalyst system in addition to the alkyl silicate and the tin salt also contains a silanol containing polysiloxane fluid. The advantage of the Smith system was that the silanol containing polysiloxane which was pre-reacted into the tin salt-alkyl silicate system resulted in a product with an extended shelf life and did not have the limited shelf life of the prior art Lewis U.S. Pat. No. 3,186,963 system.
Another disclosure which is to be referred to is the disclosure of Modic U.S. Pat. No. 3,457,214 which discloses two component RTV composition comprising a silanol polymer, a resinous copolymer composed of monofunctional siloxy units and tetrafunctional siloxy units, a reinforcing silica filler and a metal salt of carboxylic acid as a particular type of catalyst. It is stated that it was possible by such a composition to incorporate silica filler in the two component system and also to make a clear see-through mold with such a system. Accordingly, the resinous copolymer composed of monofunctional siloxy units and tetrafunctional siloxy units allows the production of a two component system. However, the composition did not cure as rapidly as would be desired. In addition, it was necessary to have solvent in the system since the resinous copolymer had first to be dissolved in a solvent before it could be incorporated into the silanol end-stopped polymer.
The improvement over such a system is disclosed in Modic U.S. Pat. No. 3,205,283 which discloses taking the mixture of silanol polymer and organic solvent solution of the resinous copolymer, mixing them together and then stripping the solvent off by heating the mixture at temperatures from about 40.degree. C. to a temperature equal to the boiling point of the solvent. However, even in such a system, it was necessary to disperse the tin salt or the other catalyst in a solvent so as to form the second component so they could be dispersed homogeneously into the first component to produce the final cured composition. Accordingly, although the disclosure of Modic U.S. Pat. No. 3,205,283 removed the solvent in the basic component of the system, it did not remove the solvent in the tin portion of the system. It should be noted that the disclosure of Modic U.S. Pat. No. 3,205,283 reduced the amount of solvent that was in the system from roughly 20% to about 4 or 5 percent. It was still desirable to remove all of the solvent from the system, since solvents create pollution problems, and it is desirable to remove all of the solvent from the system, having a completely solventless system.
The solvent is removed from the tin salt in the system of Modic U.S. Pat. No. 3,205,283. The tin cannot be properly or uniformly dispersed in the other component of the two packaged system. If the tin salt is mixed with the other ingredients, it will precipitate out from the other ingredients and will not be able to uniformly disperse into the basic component containing the silanol polymer. It should be further noted that previously there was prepared a composition containing a silanol polymer, a resinous copolymer composed of monofunctional siloxy units and tetrafunctional siloxy units which comprised one component of the two component system in which the second component comprised an akyl silicate, tin salt and a nitrogen functional self-bonding additive. However, in such a system, there was utilized a solvent in the second component so as to dissolve and uniformly disperse all the three components so as to form a uniform component. Then when it was necessary to use the system, the second component or a catalyst component would be mixed into the first component to form a system which could be applied to cure to a silicone elastomer. Accordingly, even this system was not solventless.
Two new applications have arisen for such two component RTV systems. One new application in which organics have been used previously, is to encapsulate photovoltaic cells. The organics that were used previously do not have any solvents, however, they would color or change color upon being exposed to ultra violet light for any period of time and would thus, cause interference with the passage of light into the photovoltaic cell. The other problem with organics was that such organics would be embritteled by the passage of time and by weathering, such that the organics will turn into powder and break off. Accordingly, it was highly desirable to develop a solventless silicone composition which would be utilized to encapsulate photovoltaic cells. It was also necessary that such encapsulated silicone composition be clear and not change color with the passage of time upon being exposed to ultraviolet light. Accordingly, it was highly desirable to have an encapsulant for photovoltaic cells which was a clear material and which had good adhering self-bonding properties and which would not be affected by weathering as most silicones are not and would not change color upon being exposed to ultraviolet light for extended periods of time. Another application in which it was desired to have a two component RTV composition, was a potting composition for automotative electrical components. For such a composition, it was necessary that the composition be opaque, as was desired by the customer, that the composition be of low viscosity in the uncured state such that it could fill voids and release entrapped air therefrom, upon curing and that it would have good self-bonding properties to the underlying electrical component.
Accordingly, it is one object of the present invention to provide for a two component RTV composition which is completely solventless (i.e. Hydrocarbon and aromatic solvents). It is an additional object of the present invention to provide a two component RTV composition which is completely solventless and has reasonable strength properties.
It is still an additional object of the present invention to provide for two component RTV compositions which are completely solventless and which can be utilized to encapsulate photovoltaic cells as well as encapsulate automotative electrical components.
It is yet an additional object of the present invention to provide for a process for producing a two component RTV composition which is completely solventless and is suitable as an encapsulating composition for photovoltaic cells and also as a potting composition for automotative electrical components.
These and other objects of the present invention are accomplished by means of the disclosure set forth herein below.